1. Field of the Invention
The present invention relates to an apparatus and a method for forming silica glass elements in which the shortening of the heating time of a glass material for forming is particularly aimed at.
2. Description of the Related Art
Methods for manufacturing glass elements such as glass lenses that are required to provide high accuracy are broadly divided into two: a method which involves forming optical surfaces by grinding and polishing a glass material and a method by reheat pressing. Glass elements are often manufactured by the former method.
In the former method, however, a dozen or so steps are required for forming curved surfaces by grinding and polishing. In addition, there arises a problem that large amounts of glass grinding dust harmful to workers are generated. Moreover, the former method has another problem that it is difficult to manufacture high-value added glass elements having optical surfaces of aspherical surface shape with the same accuracy and in large quantities.
In contrast, in the latter manufacturing method by reheat pressing, by the heating and pressing of a glass material having a predetermined form, the shape of dies is printed onto the glass material thereby to form a glass element such as a glass lens. For this reason, this manufacturing method has an advantage that only one step of press forming is necessary for forming curved surfaces. Furthermore, this method has another advantage that once dies are fabricated, a large number of glass elements can be manufactured according to the accuracy of the dies.
In recent years, silica glass elements have received significant attention for such reasons as small thermal expansion, small impurities and good ultraviolet transmittivity, and uses of silica glass elements are expected in the fields of optical communications and medical services. For this reason, various types of silica glass elements including those of complex shape, such as microlens arrays, and various sizes from ultra-miniature to large size have come to be required.
However, when silica glass elements are formed by the above-described manufacturing method by repeat pressing, the following problems exist awaiting solutions. In the manufacturing method by reheat pressing, a glass material is interposed between dies, the interior of a forming chamber including the dies and glass material is brought into an inert gas atmosphere or a vacuum atmosphere in order to prevent the oxidation of the dies, the glass material is then heated by a resistance heating device, a high-frequency heating device, an infrared lamp, etc. and pressed by the dies, and a formed article is taken out after cooling. When a usual optical glass is formed, the forming temperature is about 700° C. at the highest and cemented carbide alloys (WC, tungsten carbide) are generally used as die materials. However, in the case of glass materials of high melting point such as a silica glass material, which requires a high forming temperature of about 1400° C., it is necessary to use die materials which have heat resistance at high temperatures. Moreover, a silica glass material has a problem that the material devitrifies due to crystallization when exposed to high temperatures of not less than 1200° C. for a long time. Because a glass element which has devitrified cannot be used as an optical element, in reheat pressing it becomes necessary to shorten the heating and pressing time.